1. Field of the Invention
The present invention relates to an improved method of crystallizing 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]-dodecane, hereinafter referred to as CL-20. More specifically, the present invention relates to a high temperature method of crystallizing an epsilon-polymorph of CL-20.
2. State of the Art
For many existing propellant and weapons systems, a critical ingredient in terms of propulsive and explosive performance is an oxidizer or energetic filler, such as CL-20. CL-20, with its substantial increase in performance output, is an organic oxidizer/filler presenting significant opportunities in terms of energy capabilities for propellants and explosives. For example, using CL-20 as the energetic filler or propellant component in weapons systems may provide increased anti-armor penetration, enhanced missile payload velocity and flight, increased underwater torpedo effectiveness and lethality, and improved gun propellant impetus.
The performance of CL-20 in propellant and weapons systems is highly dependent upon the crystal polymorph of CL-20. CL-20 has several different crystal polymorphs including the alpha-polymorph (“α-polymorph”), the beta-polymorph, the epsilon-polymorph (“ε-polymorph”), and the gamma-polymorph. The ε-polymorph is known in the art and has high energetic performance, high density, and low sensitivity compared to the other polymorphs, which makes the ε-polymorph more desirable for use in propellant and weapons systems. However, many conventional CL-20 synthesis techniques produce the α-polymorph as the predominant crystal polymorph, which has a much lower density than the ε-polymorph. Therefore, the CL-20 synthesized by many conventional techniques must be dissolved and recrystallized to increase the proportion of the ε-polymorph.
Conventionally, CL-20 has been crystallized using a chloroform nonsolvent to precipitate the CL-20 from ethyl acetate. Chloroform has been found to produce the desirable ε-polymorph of CL-20 consistently and reproducibly. However, one disadvantage of using chloroform as the nonsolvent is that defects, such as voids and multiple crystalline shapes (agglomerates), are often found in the crystalline structure of the ε-polymorph CL-20. Another disadvantage of this conventional technique is that the chloroform and the ethyl acetate cannot be effectively and efficiently separated by distillation, which complicates their reuse. Because the chloroform cannot be easily reused, a continual discharge of a chlorinated waste stream must be disposed of in an environmentally acceptable manner. As a chlorinated compound that potentially contributes to ozone depletion, the waste disposal of chloroform and other chlorinates is complicated. Therefore, it is advantageous to crystallize CL-20 into the ε-polymorph with solvents and nonsolvents that can be recycled within the crystallization process without producing a discharge of chlorinated or halogenated waste.
A CL-20 crystallization technique that avoids the use of chloroform and other chlorinated solvents and nonsolvents is disclosed in U.S. Pat. No. 5,874,574 to Johnston et al., which describes dissolving CL-20 in a solution containing a CL-20 solvent, such as ethyl acetate. A low density CL-20 nonsolvent is then added to the dry CL-20 solvent phase to crystallize the ε-polymorph of CL-20. Nonsolvents include aromatics, such as benzene and toluene and the like, and relatively lower carbon number hydrocarbons. A drawback to this process, however, is that multiple shapes of CL-20 crystals are formed. The lack of uniformity in crystalline shape raises the viscosity of compositions that CL-20 is blended into, which adversely affects the capability to attain high solid loadings. Also, the lack of uniformity between separate production lots of CL-20 lowers predictability of explosives and propellants containing the CL-20.
Another CL-20 crystallization technique is disclosed in U.S. Pat. No. 5,973,149 to Bescond et al., in which ε-polymorph CL-20 is crystallized from a saturated solution of an organic solvent and a nonsolvent. The organic solvent is an ester, nitrite, ether, or ketone (with the exception of acetone). The nonsolvent is an aliphatic hydrocarbon, aromatic hydrocarbon, or mixtures thereof. The saturated solution is seeded with crystals of ε-polymorph CL-20 and then is concentrated by evaporation to produce CL-20 particles having particle size fractions within the 10 micron to 100 micron range. The small particle sizes of these crystals can be disadvantageous where relatively slow burn rates are desired or either bimodal or trimodal formulations are needed to tailor processability and/or ballistic properties.
To overcome these problems, it would be desirable to provide a method of producing the ε-polymorph of CL-20 that possesses a reproducible crystal shape and excellent quality in high yields. The method should be environmentally friendly and more economically efficient than known methods.